II.

Arch

An arch, in construction, is a rigid span curving upwards between two points of support. It appears in a variety of structures, such as an arcade, formed by a row of arches, supported by load-bearing arches or a roof or a bridge, or as a single, free-standing triumphal or memorial arch. The traditional stone or brick arch, formed of many segments held in place by lateral thrust, was developed to connect a greater distance between two supports than a single horizontal beam, or lintel, could bridge. Since the 19th century, arches have also been made of single, curved spans of iron, steel, or reinforced concrete.

The masonry arch has many elements. Its supports may be walls, piers, or columns, and the capstones from which it springs are known as imposts. The upper part of the arch is the crown, the portions near the impost are the haunches, its wedge-shaped segments are voussoirs, and the central or crowning voussoir is the keystone. The inner edge of the arch is the intrados; the outer edge, the extrados; and the undersurface, the soffit. The moulded band that often is found around the opening of the arch is the archivolt. The wall spaces on either side of an arch, or between adjacent arches, are spandrels. If the space between the arch and the lintel is filled in, the resulting flat surface is the tympanum.

Arches have been built since prehistoric times. Rude prototypes were made by leaning two slabs of rock together or by constructing a stepped, or corbelled, arch in which projecting elements from a wall rise in steps from the supports to meet in the centre. The Egyptians, Babylonians, and Greeks used the arch, generally for secular structures, such as storerooms and sewers. The Assyrians built palaces with arched ceilings, and the Etruscans used arches in bridges, passageways, and gates. The Romans, however, were the first to develop the arch on a massive scale. They used the semicircular arch freely in secular structures such as amphitheatres, palaces, and aqueducts, but their temples usually had the post-and-lintel construction of Greek temples. The few vaulted exceptions, however, include the Pantheon in Rome.

During the Middle Ages, Byzantine architecture in the East and Romanesque architecture in the West retained the characteristic round Roman arch. Islamic architects developed a rich variety of pointed, scalloped, horseshoe, and ogee (S-curve) arches for mosques and palaces. In Moorish Spain, horseshoe arches were set on delicate columns, giving a characteristic airy effect. Gothic architecture in Western Europe was characterized by the pointed arch, which minimized outward thrust and thus made possible higher, thinner, window-filled walls, creating the lofty, spacious interiors of Gothic cathedrals. In the 20th century, arches of moulded reinforced concrete based on the curve of a parabola have been used in all sorts of public structures.